"what is needed on the folk scene, which appears to be some sort of vetting panel that decides who should perform and who should go away quietly"

Well - that IS what happens with professional performers in any equivalent business isn't it? Yes, nothing wrong with that. If I paid money to go see musicians (or any other serious performance based art form) and incompetent tone deaf awfulness was up on stage, I'd want my money back. I see nothing wrong in managers vetting performers (be they paid or unpaid) before a paying audience.

Amateur song / music sessions are a different matter. The amateur session I regularly attend is a mixed bag, but there are still people who like to come and listen for free as is their right - who's going to tell *them* "sorry, but we're only amateurs so you can't drop in and listen, as it might put you off *proper* folk music?"